Add project restore files and NuGet cache for GsaViewer

- Created project.nuget.cache to store NuGet package cache information.
- Added project.packagespec.json to define project restore settings and dependencies.
- Included rider.project.restore.info for Rider IDE integration.

GsaEditor/ViewModels/EntryTreeNodeViewModel.cs

@@ -0,0 +1,40 @@
+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+using GsaEditor.Core.Models;
+
+namespace GsaEditor.ViewModels;
+
+/// <summary>
+/// Represents a node in the archive tree view.
+/// Can be a folder (with children) or a leaf file node (with an associated <see cref="GsaEntry"/>).
+/// </summary>
+public partial class EntryTreeNodeViewModel : ViewModelBase
+{
+    [ObservableProperty]
+    private string _name = string.Empty;
+
+    /// <summary>
+    /// Full relative path of this node within the archive (using '/' separator).
+    /// </summary>
+    public string FullPath { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Whether this node represents a folder (true) or a file (false).
+    /// </summary>
+    public bool IsFolder { get; set; }
+
+    /// <summary>
+    /// Whether this node represents a file.
+    /// </summary>
+    public bool IsFile => !IsFolder;
+
+    /// <summary>
+    /// The archive entry this node represents. Null for folder nodes.
+    /// </summary>
+    public GsaEntry? Entry { get; set; }
+
+    /// <summary>
+    /// Child nodes (sub-folders and files within this folder).
+    /// </summary>
+    public ObservableCollection<EntryTreeNodeViewModel> Children { get; } = new();
+}